The present invention generally relates to a polishing machine and method for abrading or polishing a workpiece.
The present invention also relates to a tool and an abrasive cup for fitment to a tool for the use in abrading or polishing a workpiece.
The abrading or polishing of the surface of a workpiece is a technique which has applications in many different fields including the production of semi-conductor devices and optical components. The requirement is to provide a surface which has a particular surface contour and a particular surface finish i.e. smoothness. In the field of optical polishing there are two different techniques, one technique uses a tool for polishing which has a size comparable with that of the size of the workpiece. The limitation of this technique is that the tool is designed for a specific workpiece and this cannot be used universally. In order to reduce this limitation, an active lap has been developed as disclosed in GB 2163076 wherein the pressure distribution over the workpiece can be varied in order to differentially abrade or polish.
In the second technique the tool is substantially smaller than the workpiece and is moved across the workpiece in order to carry out the abrading or polishing process. Such a technique is described in U.S. Pat. No. 4,128,968. In this technique two pads are maintained in contact with the surface of the workpiece and are relatively rotated and moved in a spiralling path around the surface of the workpiece. Another such technique disclosed in WO97/00155 uses a tool which has a flexible working surface so that the effective area of contact with the workpiece can be controlled. This provides the benefit that the area being polished at any one time during the polishing cycle can be controlled.
In these prior art techniques, the tool is usually spun around an axis normal to the workpiece. A limitation to this technique is that on the axis the relative movement is zero and thus the removal or ablation rate is zero. Thus the use of such a tool having such a removal prof lee Bakes it difficult to achieve a desired target profile using an automatic polishing or abrading technique.
In the technique of WO97/00155, the angle of attack of the tool to the workpiece is variable using an arrangement which provides a xe2x80x9cvirtual pivotxe2x80x9d on the surface of the workpiece. This has the benefit of ensuring when the tool is tilted there is no lateral or vertical movement of the centre of the tool in contact with the workpiece. However, the disclosed mechanical arrangement is complex and large.
U.S. Pat. No. 4,958,463 discloses a different technique in which a resilient working member is rotated about its axis parallel to the surface of the workpiece to provide relative lateral motion between the working member and the surface of the workpiece. The working member is held to rotate in a mounting member. The mounting member mounting the working member is also rotated perpendicularly to the surface of the workpiece. While this technique does not suffer from the lack of removal of material in the centre of the area of contact, it requires a complex arrangement involving the use of two motors to provide the two axes of rotation.
In accordance with a first aspect of the present invention there is provided a method and apparatus for abrading or polishing a workpiece. A workpiece is held on a holding surface of the and a head having a surface for abrading or polishing the workpiece is moved across the workpiece in a figuring pattern in order to polish or abrade the workpiece. In addition to the figuring movement of the head, the face of the head carrying out the abrading or polishing is arranged to move in a direction laterally on the surface of the workpiece by inclining and rotating the head. The direction of lateral movement of the face is rotated by moving the head to positions which are a precession of the inclined head about a precession axis normal to the workpiece surface.
Thus in accordance with this aspect of the present invention, instead of having a removal profile which is zero at the centre of the area of contact of the tool, it is possible to simply incline the head to use a face which is moved laterally relative to the workpiece, thus providing a non-axially symmetric removal profile at any one instance in time. An axially symmetric removal profile is however an advantage when deterministic automatic polishing is required. In order to make the average removal profile over a period of time symmetric, the head is moved to positions precessed relative to the surface of the workpiece such that the direction of lateral relative movement of the face of the tool rotates. Thus any pattern generated in the surface of the workpiece by the lateral movement at an instance in time will be generated at a number of rotational angles thus reducing the defects and producing an axially symmetric profile.
Another advantage of the technique of this aspect of the invention is that the movement of the abrading face is self-wetting. The cooling/lubricating fluid or slurry used between the abrading surface and the workpiece will be carried under the tool by the polishing action. In contrast, in the prior art techniques which use axial rotation of a tool normal to the surface of the workpiece, the cooling/lubricating fluid will tend to move to the circumference of the polishing area by centrifugal force.
This aspect of the present invention is applicable to any form of inclined rotating tool which can provide relative lateral movement between the workpiece and the abrading face and which can be precessed to rotate the direction of lateral movement e.g. an axially rotatable conical shaped tool.
In order to achieve averaging, preferably the precession takes place throughout at least 360 degrees. This can be achieved by incrementing the precession. Preferably such increments should be over more than one precession cycle. In one embodiment the increment in the precession is not an integer division of 360 degrees so that the direction of relative lateral motion is different for each cycle. In another embodiment the increments are symmetric about the 360xc2x0 precession cycle.
In a preferred embodiment, the face comprises a compliant bulbous portion extending from the head. As the compliant portion is rotated about its inclined axis the bulbous portion forms an area of contact with the workpiece wherein there is relative lateral movement.
The abrading face can comprise a cloth or pitch on to which an abraded loaded slurry e.g. a diamond paste is placed. Alternatively, a bound abrasive can be used which is bound to the abrading face. When such a bound abrasive is used, only a cooling/lubricating fluid is required.
A second aspect of the present invention provides a polishing or abrading machine or method wherein a head carrying a face for abrading or polishing is held by a mechanical arrangement to allow for figuring of a workpiece. The mechanical arrangement includes a tilting mechanism arranged to tilt the head about a pivot to enable tilting relative to the workpiece. In this way not only can the head follow the contours of the workpiece but also it can be tilted to either follow the surface or be inclined at a required angle to the surface. Because the pivot is not on the workpiece when the head is tilted there is a displacement of the face of the head across, to or away from the workpiece. This is compensated for by either calculating or looking-up the required compensation values to control the mechanical arrangement to take up the displacements in the lateral and vertical directions.
This aspect of this present invention differs from the prior art disclosed in WO97/00155 in that a far simpler pivoting arrangement can be provided for example by use of an orthogonal arcuate track arrangement allowing tilting in any solid angle. In order to enable the use of a such a simple pivoting arrangement however, the displacement of the polishing face must be compensated for by a controller which determines the solid tilt angle and compensates the lateral and vertical displacements accordingly. Further, even if the virtual pivot is used as disclosed in WO97/00155, if a soft face is used e.g. a compliant material, it is necessary to compensate for the displacement of the compliant material as the head is pressed onto the workpiece.
Either the first aspect or the second aspect of the present invention can be implemented using a polishing apparatus under computer control. These aspects of the present invention can thus be embodied as a computer program and a carrier medium storing the computer program for controlling a processor to control the polishing apparatus. Since the computer program can be transmitted over a network such as the Internet, these aspects of the present invention can be embodied as a signal carrying the computer program for controlling the processor to control a polishing or abrading apparatus.
A third aspect of the present invention provides an improvement to the soft tool of WO97/00155. The soft tool is provided with an abrasive cup for releasable fitment to the soft tool wherein a sheet is preformed in the shape of the surface of the soft tool to be used for polishing and is sufficiently flexible to allow its deformation as a result of compression of the soft tool during abrading or polishing. The sheet is held at its periphery by a carrier member which is releasably mountable to a holder of the soft tool.
Since the sheet will become worn during the polishing or abrading process, it is removable from the tool. Since the surface of the tool to be used for polishing is compliant, the membrane must be flexible to conform to any compliance of the flexible tool Because of the compliance of the soft tool and the need for the sheet to flex to follow the compliance, it is preferred that means are provided to allow relative lateral movement between the sheet and the surface of the compliant tool. This is necessary in view of the different radii of curvature of the surface of the soft tool and of the sheet. Suitable means which can allow for some relative lateral movement comprises a lubricant or an unset adhesive. Use of an Unset adhesive provides the advantage that the sheet is adhered to the surface of the soft tool and thus benefits from the support provided thereby. In other words during the abrading or polishing operation some of the lateral force experienced by the sheet can be passed to the soft tool. If on the other hand a lubricant is used, the sheet must have sufficient torsional strength to be able to withstand the forces experienced during the abrading or polishing operation as the tool is dragged across the surface of the workpiece since the sheet is mechanically driven from its periphery
A fourth aspect of the present invention provides a method and a machine for abrading or polishing a workpiece and which has a soft tool head comprising a fluid filled chamber which is detachable from a tool body without having to break the fluid seal. The tool body is a rotational body which extends along the rotational axis and has a pressure transmission means at one end thereof for transmitting pressure to the fluid chamber of the tool head. The tool head is releasably mounted on the tool body and comprises a head housing, a head fluid transmission means and resilient membrane forming the seal fluid chamber. The head fluid pressure transmission means is arranged to cooperate with the pressure transmission means of the tool body to transmit pressure to the fluid in the head of fluid chamber. The resilient membrane is held at its periphery by the head housing to extend in a curved manner therefrom for the application of pressure to the workpiece during abrading or polishing.
This aspect of the present invention provides a resilient working member, the resilience of which can be controlled by controlling the pressure of the fluid within it. The tool head benefiting from this advantage can also be readily interchanged when necessary due to wear or due to the need to change to different tool head sizes
In one embodiment of the present invention, the tool body has a fluid chamber filled with fluid terminating at the pressure transmission means. This enables fluid pressure to be transmitted through the fluid body chamber to the head body chamber from a fluid pressure control arrangement provided separately into the rotatable tool. Because two separate fluid filled chambers are used in the tool head and the tool body, the tool head can be readily removed front the tool body without breaking the fluid seal.
In one embodiment the fluid pressure is transmitted from the body fluid chamber to the head fluid chamber via respective displacement devices mounted on the tool body and the tool head respectively. The respective displacement devices are coupled to one another to provide the transmission of pressure. This may be a direct physical coupling or a coupling via an intermediary e.g. via air.
A fifth aspect of the present invention provides a method and apparatus for controlling polishing or abrading of a workpiece. Data defining an influence function of the tool is used. The influence function defines the pattern of removal of material from the workpiece for a predetermined dwell time or speed of the tool. The desired profile is compared with the current profile of the surface to the workpiece and a difference between them is determined. Dwell times or tool speeds for predetermined positions on the surface of the workpiece are determined using numerical optimisation of the dwell times or tool speeds for the predetermined positions using the influence function to reduce a cost function.
The technique is preferably iterative wherein a cost function is repeatedly determined for various dwell times or tool speeds until a minimum cost function is found relating to the optimum dwell times or tool speeds to substantially achieve the desired profile.
Thus for this technique, predetermined positions are used for the application of the influence functions and the technique thus becomes simply the optimisation of a set of values to achieve a result.
There are many techniques which can be used for this type of optimisation. The technique can reduce the sum of the squares of the difference between the target removal profile and a predicted removal profile. A genetic algorithm can be used in order to determine candidate values for the dwell times or tool speeds. A cost function can be calculated for the dwell times or tool speeds to determine whether the proposed value is a candidate for keeping in the xe2x80x9cgenexe2x80x9d pool, or not.
In one embodiment, the desired profile comprises a desired radial profile for a circular workpiece and is thus only a two dimensional profile. This radial function applies equally to all radii of the surface of the workpiece. The influence function is also defined as a two dimensional function and the predetermined positions comprise radial positions across the surface of the workpiece. Thus the numerical optimisation technique only comprises the optimisation of a sequence of numbers defining the dwell times for radial positions across the surface of the workpiece, having regard to the radial error in the profile achieved.
In a more complex embodiment of the present invention, the desired profile is defined across an area of the surface of the workpiece and thus comprises a three dimensional profile. The influence function is thus also necessarily defined as a three dimensional function or at least a projection of a two dimensional radial function into three dimensions. The predetermined positions at which the influence function is to be applied comprise a two dimensional array of positions across the surface of the workpiece. Thus in this embodiment of the present invention, the numerical analysis technique must determine dwell times or tool speeds for the two dimensional array of positions across the surface of the workpiece. In order to reduce computational time, it is possible to use a coarse grid of array positions in order to determine the dwell times or tool speeds for the positions. Dwell times or tool speeds for intermediate positions can then be determined by interpolation where necessary. For example, when the figuring pattern to be traced out by the head comprises a circular pattern, the interpolation takes place in order to define dwell times or tool speeds along arcs to be traced by the head. Where the figuring pattern is to comprise a rastering scan, the interpolation is carried out for the linear rastering pattern in order to determine dwell times or tool speeds along the path to be taken by the head during the figuring operation.